Add PLATFORM_EXPORT to Layout{Point,Size} classes.

During debugging use of toString() is often helpful. PLATFORM_EXPORT is required
for the method implementations to be available to some callsites at link
time. Similar classes in this package already have PLATFORM_EXPORT on the class
declaration.

TBR=pdr

Review-Url: https://codereview.chromium.org/2153633002
Cr-Commit-Position: refs/heads/master@{#405667}
2 files changed