Only use fontMetrics if available in line-height computation

Only use fontMetrics if available in line-height computation

Change ComputedStyle::computedLineHeight to check if primaryFont is non
null before calling fontMetrics().lineSpacing() as metrics requires the
font to have been loaded. This is a speculative fix for a top crasher.

BUG=542338
R=pdr@chromium.org

Committed: https://crrev.com/37e42dda0182625f8bcb7277a5802ca6fbe32890
Cr-Commit-Position: refs/heads/master@{#354686}

[pdr., 2015-10-16 22:52:22]

On 2015/10/16 at 22:42:24, eae wrote:
> 

Codechange LGTM.

Would you be up for adding a test though?

[eae, 2015-10-16 22:54:57]

Sadly not, haven't even been able to reproduce the problem. Going entirely by
minidumps here :(

[eae, 2015-10-16 23:08:13]

Patchset #1 (id:1) has been deleted

[pdr., 2015-10-16 23:41:52]

On 2015/10/16 at 22:54:57, eae wrote:
> Sadly not, haven't even been able to reproduce the problem. Going entirely by
minidumps here :(

LGTM

[pdr., 2015-10-16 23:41:58]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-10-16 23:43:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1410173002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1410173002/20001

[commit-bot: I haz the power, 2015-10-17 00:08:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-17 00:08:17]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[eae, 2015-10-17 17:02:37]

The CQ bit was checked by eae@chromium.org

[commit-bot: I haz the power, 2015-10-17 17:02:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1410173002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1410173002/20001

[commit-bot: I haz the power, 2015-10-17 18:05:58]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-10-17 18:06:51]

Patchset 1 (id:??) landed as
https://crrev.com/37e42dda0182625f8bcb7277a5802ca6fbe32890
Cr-Commit-Position: refs/heads/master@{#354686}